In operation, the blades in gas turbines, particularly in aircraft engines, are subject to wear caused by erosive particles, for example sand or dust.
In the forward section of the gas turbine, the compressor section, erosion generally brings about a flattening of the leading edge of the compressor blades. Here, the compressor blades of the first compressor step, the so-called “fan blades”, are many times larger than other blades of the propulsion unit. In the unworn state, the compressor blades have an aerodynamically optimized profile, which is adversely altered due to the wear.
As a consequence of this type of deterioration of the aerodynamic profile, the gas turbine becomes less efficient, that is, the gas turbine requires more fuel for the same performance.
From an economic point of view, therefore, counteracting the wear of compressor blades, particularly a deterioration of aerodynamically optimized blade geometries, is worthwhile.
For this purpose, repair processes have been established which are geared towards providing damaged compressor blades with as aerodynamically optimal as possible geometries again.
A device is known from U.S. Pat. No. 6,302,625 B1, with which the blade-edge can be reconstructed. During the reconstruction process, this device is directed towards the blade in a predetermined direction by means of various positioners.
The positioners form a type of positive guide towards the blade for the device, such that the device can be easily managed by touching down on the blade edge and moving along the blade edge.
The positioners are arranged on a lateral surface of a conically tapered slot, with which the device is placed onto one of the edges of the blade. When the device is suspended on the blade edge, the positioners arrive at one of the blade surfaces laterally to the attachment. The placement depth of the device on the blade edge is limited by the conicity and the depth of the slot of the device. The actual section which changes the shape of the blade is arranged in the base of the slot.
Hence, the conical slot forms a guide for the device, whereby the depth to which the device is placed on the blade is dependent on the individual management of the device. Since the recontouring of the blade takes place by means of processing the surface, it is also necessary to have a certain minimum clearance between the blade edge and the base of the slot in order to form a cavity in which excess material can be collected. Owing to this minimum clearance, the device cannot be guided with positional accuracy in relation to the depth to which the blade edge projects into the slot.
Moreover, some gas turbine blades, for example new types of fan blade, are furnished with geometrically complex edge geometries which complicate the precise machining of the blade edges using common devices.
For example, for certain fan blades, the blades in the upper section work in the supersonic range during operation and the lower blades work in the subsonic range. For this reason, the contour of the blade is changed, both in pitch angle and in cross section, and presents a complex geometric edge geometry which can only be precisely machined with difficulty.
Furthermore, the base material of the gas turbine blade is always worn away due to the cutting machining used routinely and hence the chord length of the blade is shortened by the recontouring process.